1. Field of the Invention
The present invention relates to active filtering, more specifically tuning of active filters using operational amplifiers as a gain stage.
2. Background Art
A hard disk drive is storage device that uses magnetically coated disks called platters for the storage of digital data. The terms “hard disk,” “hard disk drive,” and “hard drive” are all used interchangeably, because the disk and its corresponding drive mechanism are a single unit. Most hard disk drives contain at least two platters dependent upon the storage capacity of the hard disk drive. Hard disk drives with a larger storage capacity contain a greater number of platters.
Each platter has a smooth magnetic surface for the storage of digital data. Data is written to a platter by applying a magnetic field from a read-write head close to the magnetic surface of the platter. The magnetic medium on the surface of the platter changes its magnetization due to the magnetic field of the read-write head. The data may be read back by a magnetoresistive read sensor also located on the read-write head. The magnetoresistive read sensor changes resistance to detect the magnetic flux corresponding to bit transitions of the digital data stored on the platter.
The read channel encodes and decodes the data from the read-write head. The read channel detects bits as an analog signal from the read-write head and converts them into digital form. Read channels use advanced mixed-signal and digital-signal processing technologies, in addition to advanced data-encoding schemes and digital filtering to optimize data detection. A read channel contains a low pass filter for anti-aliasing purposes as well as band limiting noise and equalizing the read signal.
Conventional low pass filters use a Gm/C approach whereby the frequency characteristics of the filters are determined by transconductance amplifiers and capacitors. Because transconductance amplifiers are not stable and fluctuate over temperature and voltage, continuous tuning is required. If not continuously tuned, the read channel will generate corrupted data. To provide this continuous tuning, a replica of the low pass filter or a portion of the low pass filter is typically fabricated onto the same semiconductor substrate as the low pass filter. As a result a large portion, up to one quarter, of the substrate allocated for the low pass filter is used for tuning the transconductance amplifiers. In addition, the circuitry for the continuous tuning of the low pass filter consumes additional power.
What is needed is a low pass filter that is stable over temperature and voltage so as not to require continuous tuning.